Abstract:
The main aim of this study was to develop rain fall run off and sediment yield relationship for
upper didessa water shed and also asses the spatial distribution of sediment for the catchment
by using a physical process based watershed scale model, soil water assessment tool (SWAT).
The SWAT model was used large number of input parameters to simulate flow and sediment
yield of the watershed. The semi-automated Sequential Uncertainty Fitting (SUFI2)
calibration process built in SWAT calibration. SWAT-CUP were used to calibrate the model
parameters using time series of flow and sediment data of 1992 to 2003 and validated with the
observed data from years 2004 to 2007. The performance of the model was evaluated using
graphical methods to assess the capability of the model in simulating the runoff and sediment
yield for the study area. The coefficient of determination (R2) and Nash Sutcliff efficiency
(NSE) values for the monthly flow calibration using SUFI2 are 0.90 and 0.88 respectively and
it was 0.77 and 0.73 for validation. For monthly sediment yield by using SUFI2 calibration
technique the model evaluation coefficients R2 and NSE for calibration was computed as 0.88
and 0.86 respectively, for validation it was 0.78 and 0.74 respectively. The rain fall run off
and sediment yield relationship of the catchment has been determined for each sub basin of
the watershed. And also the spatial distribution of sediment yield, temporal distribution of rain
fall runoff and sediment yields of the catchment was determined and the area which is
sensitive soil erosion is identified during the study and the model result indicates that
simulated annual average sediment yield for the catchment was 10.5 t/ha/yr.
